The lockdown is providing a trial run of the “summer minimum” challenges of operating the grid as it will be in the 2030s and 2040s. Demand was historically low and renewable generation historically high on both the distribution and the transmission grids. Not only did this mean that electricity flows through the transmission grid dropped severely, but also inertia dropped to levels that required extensive intervention.

This reinforces the need for Storelectric’s CAES to provide not only absorption of renewable energy when it exceeds demand, but also the real inertia and other related grid stability services that are currently being provided by gas-fired power stations.

 Minimum Energy Flows

Readers may recall that in their Future Energy Scenarios 2015, National Grid declared that to keep the grid out of black-start conditions they need at least 5GW flowing through it. Based on the distributed solar generation that they thought existed at the time, they believed that they risked dropping below the critical 5GW figure by 2035 in the worst-case scenario. However they had underestimated solar by 3GW and omitted to consider distribution-connected wind, so the first time they hit the 5GW threshold was actually in August that same year.

During the COVID-19 Lockdown, demand has plummeted as a small increase in domestic consumption has fallen far short of a huge decrease in industrial and infrastructure (e.g. trains) consumption. Simultaneously the weather was perfect for renewable generation for weeks on end, with unseasonably strong sunshine combining with persistently strong winds. As a result, afternoon minima dropped below overnight minima on the transmission grid.

Inertia and Grid Stability

As fossil fuelled power stations continued closing during the last decade, it became apparent that they were providing many more services than just energy. Prime among these was inertia, with related capabilities such as real reactive power / load, ROCOF (Rate Of Change Of Frequency) and Phase Locked Loops – and many that are still more esoteric.

What is inertia? In a car, if the engine fails the weight of the car provides the momentum that means that it slows gently, enabling it to come to a save halt. Those in the car are protected. Without that momentum, the slow-down following the engine fault would be like hitting a brick wall. Inertia is momentum for rotating machines: power stations, being large rotating machines, have it in abundance whereas DC connected systems (including solar and wind generation, and interconnectors) don’t.

Therefore as power stations close, replaced by DC-connected generation, inertia drops alarmingly. The fundamental cause of the black-outs across the UK on 9th August 2019 was that two initial trips, one in a power station and the other in a wind farm, turned into a cascade of subsequent trips around the country because there was insufficient inertia on the grid.

And inertia is dropping very fast: this graph shows how it’s decreased over the last decade 2009-19, and is expected to continue to decrease 2020-29. National Grid has initiated many investigations and activities into these, including a number of new contract types and pathfinder activities (in the yellow box on the right of the first link) in order to contract for these system stability services. All versions of Storelectric’s CAES can provide all these services 24/7, regardless of whether charging, discharging or neither.

Consequences for Grids

Therefore National Grid had to undertake a number of actions to preserve grid stability. These include:

  1. Pay to turn 1GW contracted interconnector imports to 2.4GW exports
  2. Pay for 1.8GW curtailment
  3. Pay to turn down nuclear
  4. Pay to turn on 4.4GW power station generation

Not only is all of this activity exceedingly expensive, but also it increases the grid’s emissions. All of this would be completely unnecessary if there were sufficient of Storelectric’s CAES on the grid.

And in the 2030s and 2040s?

By the 2030s and 2040s, more power stations will have closed – they may not even be available to be turned on if needed. Interconnectors and DC connected generation will have at least doubled in importance. this situation will be much more frequent – indeed, it is likely to be the normal state of affairs in summer, and quite usual in spring and autumn.

To the above “summer minimum” issues can be added the lack of electricity on the grid when renewables are not generating sufficient for demand and interconnectors cannot make up the shortfall, such as after sunset on a windless winter evening, or during weather patterns that extend such low-generation period to days or weeks – which will also be a frequent occurrence by the 2040s according to most European countries’ energy transition plans. These are the subject of a separate brief.